1. Field of the Invention
This invention relates to the regeneration of ceramic honeycomb structures such as diesel particulate filters, and in particular, to a method for regenerating a ceramic honeycomb body that includes treatment of particulate mass, such as carbon soot in diesel particulate filters.
2. Description of Related Art
In an attempt to reduce atmospheric pollution, many countries are imposing increasingly stringent limits on the composition of exhaust gases produced by internal combustion engines and released into the atmosphere. The primary harmful substances from diesel engines, apart from small amount of hydrocarbons and carbon monoxide, are nitrogen oxides (NOx) and particulate matter.
Heretofore, many methods have been proposed in an attempt to reduce or minimize the quantity of particulate matter present in the exhaust gases emitted into the environment. Once such one widely utilized method is the placement of a diesel particulate filter or soot trap, in the exhaust system associated with the engine. Generally, a particulate filter consists of parallel channels with porous walls that are obstructed alternately. Specifically, these filters typically comprise honeycomb structures having traverse cross-sectional cellular densities of approximately 1/10 to 100 cells or more per square centimeter, and have several uses, including solid particulate filter bodies and stationary heat exchangers. Such uses require selected cells of the honeycomb structure to be sealed or plugged by manifolding and the like at one or both of the respective ends thereof. The manufacture of various honeycomb structures from plasticized powder batches comprising inorganic powders dispersed in appropriate binders is well known. U.S. Pat. Nos. 3,790,654; 3,885,977; and 3,905,743 describe extrusion dies, processes, and compositions for such manufacture, while U.S. Pat. Nos. 4,992,233 and 5,011,529 describe honeycombs of similar cellular structure extruded from batches incorporating metal powders.
As an example, FIG. 1 shows a well-known solid particulate filter body 10. The filter body 10 includes a honeycomb structure 12 formed by a matrix of intersecting, thin, porous walls 14 surrounded by an outer wall 15, which in the illustrated example is provided in a circular cross-sectional configuration. The walls 14 extend across and between a first end 13 that includes a first end face 18, and a second end 17 that includes an opposing second end face 20, and form a large number of adjoining hollow passages or cell channels 22 which also extend between and are open at the end faces 18, 20 of the filter body 10. To form the filter 10 (FIGS. 2 and 3), one end of each of the cells 22 is sealed, a first subset 24 of the cells 22 being sealed at the first end face 18, and a second subset 26 of the cells being sealed at the second end face 20. Either of the end faces 18, 20 may be utilized as the inlet face of the resulting filter 10. In a typical cell structure, each inlet cell channel is bordered on one or more sides by outlet cell channels and vice versa. Each cell channel 22 may have a square cross section or may have other cell geometry, e.g., circular, rectangular, triangular, hexagonal, etc. Diesel particular filters can be made of ceramic materials, such as cordierite, aluminum titinate, mullite or silicon carbide.
In operation, contaminated fluid is brought under pressure to an inlet face (either of the end faces 18, 20) and enters the filter 10 via cell channels 22 which have an open end at the given inlet face. Because these cell channels 22 are sealed at the opposite end face, i.e., the outlet face of the body, the contaminated fluid is forced through thin porous walls 14 into adjoining cell channels 22 which are sealed at the inlet face and open at the outlet face. The solid particulate contaminant in the fluid, which is too large to pass through the porous openings in the walls 14, is left behind and a cleansed fluid exits the filter 10 through the outlet cell channels 22.
The particulate matter captured by the particulate filter must occasionally be removed therefrom in order to preserve the performance of the filter, and as a result the performance of the associated engine, as well as to help prevent destruction of the particulate filter in the event of self-priming and uncontrolled combustion of the particulate matter trapped within the particulate trap. For example, as large amounts of particulate matter accumulate within the particulate filter, particular driving conditions can cause a trigger of “critical” regeneration, consisting of sudden and uncontrolled combustion of the trapped particulate matter. As a result, high temperatures can be generated inside the channel matrix of the particulate filter causing damage thereto.
It is therefore beneficial to periodically remove the particulate matter which has accumulated within the trap by performing a regeneration process. As noted, regenerations are a necessary process for a wall flow DPF technology to avoid engine damage and fuel efficient engine operation by eliminating high back pressure and maintaining effective filtration performance. Regeneration typically involves a means of combusting the particulate matter which has accumulated within the filter. This process typically comprises burning the particulate matter or soot, consisting mostly of carbon, that is in contact with the oxygen present in the exhaust gases. However, this particular reaction takes place naturally only at temperatures higher than about 600° C., which is significantly higher than those temperatures measured at the intake of the particulate filter in a normally functioning engine. It is therefore necessary to create conditions resulting in the regeneration of the filter by burning of the associated particulate matter. Many methods have been proposed and/or used in order to increase the temperature of the exhaust gases at the intake of the particulate filter to trigger a regeneration thereof.
Two types of regeneration processes are generally employed, including passive regeneration and active regeneration. Passive regeneration occurs when the engine produces filter inlet temperatures above 250° C. and enough NO to result in soot oxidation by NO2. A catalyst is required to convert NO to NO2 to support passive regenerations. Typically, the passive regeneration window is restricted to between 400° C. and 450° C., as the NO2 effect is limited by thermal dynamic equilibrium. Active regenerations are forced regenerations which cause the filter inlet temperature to rise to a range of higher than 500° C., thus resulting in a burnout of the majority of the carbon soot with oxygen contained within the exhaust gases. Many engines utilize fuel and diesel oxidation catalysts located upstream of the particulate filter to achieve temperatures as hot as 650° C. The diesel oxidation catalyst is a monolithic substrate without any plugs therein and that utilizes HC and O2 to generate heat. Still other engines use a burner system to generate the heat for active regenerations. The filters are commonly catalyzed with an oxidation catalyst to improve regeneration performance, i.e., to achieve lower regeneration temperatures, as well as to reduce emissions from the soot oxidation.
Soot oxidation may occur when the exhaust gas comprises a requisite amount of soot and oxygen at a high energy temperature. Regeneration with oxygen requires temperatures of greater than 500° to 550° C. to gain a significant portion of soot oxidation. However, when temperatures reach 550° to 650° C. there is an increased risk of an uncontrolled soot oxidation resulting in rapid heat release and therefore high exo-therms. Typical conditions resulting in uncontrolled regenerations are obtained for soot overloaded filters or regeneration conditions risking fast kinetics, i.e., high initiation temperatures and gas composition favorable to oxidation, and insufficient heat removal by absorption, convection or conduction. Uncontrolled regenerations risk damaging the filter by melting and cracking.